This invention relates to a fixed wavelength absolute distance interferometer. A two laser frequency chirped absolute distance interferometer (ADI) was developed so as to eliminate or reduce errors or ambiguity included in measurement results measured by a conventional (e.g., Michelson, Twyman-Green, or Mach-Zehnder type) laser interferometer (U.S. Pat. No. 7,292,347).
To find an absolute distance of a sample surface with high resolution, a coarse-medium-fine measurement scheme is carried out in the ADI, where the coarse measurement is used to remove the 2π phase ambiguity from the medium measurement, which is in turn used to remove the 2π ambiguity from the fine measurement.
The above ADI was capable of sub-wavelength interpolation, with nanometer resolution, over a potential absolute range of 1 m or greater.